Aluminum film for semiconductive devices

ABSTRACT

A semiconductor contact structure formed by a method that deposits an aluminum film limiting the growth of voids and notches in the aluminum film and forms an aluminum film with a reduced amount of voids and notches. The first step of the method is to form an underlying layer upon which is deposited an aluminum film having a first thickness. The surface of the aluminum film is then exposed to a passivation species which coats the aluminum grains and precipitates at the grain boundaries so as to prevent grain movement. The exposure of the aluminum film to the passivation species reduces void formation and coalescence of the voids. An aluminum layer having a second thickness is then deposited over the initially deposited aluminum layer. In a second embodiment of the invention, the passivation species is deposited with MOCVD and to form an electromigration-resistant alloy. A third embodiment involves multiple depositions of aluminum, with exposure to a passivation species conducted after each deposition. Each deposition is also conducted at a successively lower temperature than the prior deposition.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to the formation of aluminum films. Moreparticularly, the present invention relates to the formation of aaluminum film with a reduced occurrence and size of voids in a processsuitable for forming contacts, vias, and metal interconnect linessituated on a semiconductor substrate assembly.

2. The Relevant Technology

Integrated circuits are manufactured by an elaborate process in which avariety of different semiconductor devices are integrally formed on asmall silicon wafer. Conventional semiconductor devices includecapacitors, resistors, transistors, diodes, and the like. In advancedmanufacturing of integrated circuits, hundreds of thousands ofsemiconductor devices are formed on a single wafer.

One step in the manufacture of integrated circuits is to form metalinterconnect lines between the discrete semiconductor devices on theintegrated circuit. The metal interconnect lines allow for an electricalcurrent to be delivered to and from the semiconductor devices so thatthe integrated circuit can perform its intended function.

The metal interconnect lines generally comprise narrow lines of aluminumetched from an aluminum film. Aluminum is typically used because it hasrelatively low resistivity, good current-carrying density, superioradhesion to silicon dioxide, and is available in high purity. Each ofthese properties is desirable in contact lines since they result in afaster and more efficient electronic circuit.

A further step in which aluminum films are used and which is frequentlyconducted in the manufacturing of integrated circuits is the formationof inter-level interconnect structures between nonadjacent layers on theintegrated circuit wafer. These inter-level interconnect structuresinclude contacts, plugs, and vias. An example of an interconnectstructure in the form of a contact is shown in FIG. 1. In FIG. 1, acontact is shown with a contact opening 14 extending through apassivation layer 12 down to a silicon substrate assembly 10. Asubstrate assembly is intended herein to mean a substrate having one ormore layers or structures thereon. Contact opening 14 is filled with atitanium nitride diffusion barrier 15 and a film of aluminum 16.

One complication arising from the use of aluminum films is thephenomenon referred to as "void formation." Void formation is a processin which minute voids 18, shown in FIG. 1, form between grain boundariesas grains move and grow as a result of heat treatments following metaldeposition. Void formation is believed to stem from the solid phasegrowth of aluminum crystals as the film becomes thicker duringdeposition. Crystal growth causes grain boundary movement and gives riseto voids and notches at the interface between two adjacent grains. Thevoids, also known as "notches," coalesce at flux divergence sites suchas grain boundary triple points of a metal interconnect line, contact,or via. Voids cause a heightened resistivity in the aluminum film,thereby reducing the effectiveness of the aluminum film in providingelectrical communication.

Void formation is accompanied by a phenomenon known as electromigration.Electromigration occurs as an electrical current flows through analuminum portion of an interconnect line, contact, via, or otherstructure. When a voltage is applied across the aluminum, electronsbegin to flow through the aluminum. These electrons impart energy to thealuminum atoms sufficient to eject aluminum atoms from their latticesites. As the aluminum atoms become mobile, they leave behind vacancies.In turn, the vacancies are also mobile, since they can be filled byother aluminum atoms which then open new vacancies. In the phenomenon ofelectromigration, the vacancies formed throughout the aluminum line tendto coalesce at the grain boundary triple points of the metal line,thereby also helping to form voids.

In a further complication, voids 18 tend to coalesce due to increasedmovement of the grains at higher temperatures to form large-scale voids20, as shown in FIG. 2. Large-scale voids 20 severely limit conductivityand can cause failure of the interconnect, via, or contact. This canresult in the failure of the semiconductor device as a whole and failureof the entire integrated circuit. This problem occurs more frequentlyand is increasingly problematic at greater miniaturization levels due tothe smaller size of the structures in relation to large scale voids.

In one attempt to eliminate void formation, aluminum is mixed with othermetals to form an aluminum alloy. For example, copper is frequentlyadded to aluminum. The addition of copper increases the energy requiredto cause the voids to form in the metal interconnect line. Titanium andother metals are also frequently deposited together with the aluminum,which metals arc then alloyed to the aluminum in a high temperatureanneal process flow step. Alloying is only a partial remedy, however, asvoid formation still continues to occur over time, especially as thesize of aluminum features decrease.

To ensure adequate coverage of aluminum films, which is desirable infilling high aspect ratio VIA and contact holes, and in maintaining lowcontact resistance and consistent etching, the prior art has turned tocold wall CVD aluminum deposition processes. One example of this isorganometallic chemical vapor deposition (MOCVD) using dimethyl aluminumhydride (DMAH). When so doing, a base layer of titanium nitride istypically deposited by chemical vapor deposition prior to the depositionof the aluminum film. The base layer of titanium nitride assists in theprocess of achieving uniform nucleation of the aluminum film. While thisprocess is somewhat beneficial, the deposited aluminum films continue toexhibit voids 18 in aluminum film 28 and at the aluminum-titaniumnitride interface due to grain growth, an illustration of which appearsin FIG. 3.

FIG. 3 shows a substrate 10 subjacent to a passivation layer 22 composedof, for example, BPSG or SiO₂. A titanium nitride base layer 26 isformed over passivation layer 22 and is subjacent an aluminum film 28. Aplurality of voids 18 have formed in aluminum film 28 and at aninterface 24 between titanium nitride base layer 26 and aluminum film28. Voids 18 result in increased resistance in aluminum film 28,reliability problems of integrated circuits being formed by thestructure of FIG. 3, and ultimately device failure that results inlowered yield rates of the process by which the integrated circuits aremanufactured.

As a result of the above discussion, it is apparent that a need existsin the art for a method of aluminum film deposition which can be used toform uniform aluminum films with substantially eliminated or arrestedvoid formation. Such a method would be additionally beneficial if itcould be used for forming aluminum interconnect lines and inter-levelinterconnect structures, if it could be used with alloying processes,and if it could provide reduced electromigration.

SUMMARY OF THE INVENTION

In accordance with the invention as embodied and broadly describedherein in the preferred embodiment, a method is provided for depositingaluminum films with a reduced occurrence and size of voids and notches.The method comprises in a first step, forming an underlying layer. Onepreferred underlying layer is titanium nitride which is deposited in aconventional manner.

In a further step, aluminum of a predetermined first thickness isdeposited over the titanium nitride underlayer. The first thickness ofaluminum is typically between about one third and one half of the totaldesired thickness of the aluminum film being deposited. The aluminum ispreferably deposited with a cold wall chemical vapor deposition process,but could be deposited using any suitable process. The surface of thefirst thickness of aluminum is then exposed to a passivation specieswhich coats the aluminum grain and precipitates at the grain boundaries.The passivation species comprises compounds such as O₂, N₂, TiCl₄,TDMAT, SiH₄, GeH₄, as well as members of the halogen and interhalogenfamilies of gases. Other species which achieves surface passivation canalso be used.

In a further step, an aluminum film having a second thickness isdeposited over the aluminum film having the first thickness. In thebasic embodiment, the second thickness of the later deposited aluminumfilm comprises the remainder of the final thickness of the aluminum filmdesired to be formed.

The passivation species to which the surface of the aluminum film havingthe first thickness is exposed causes friction between the aluminumgrains, both chemically and mechanically, so as to arrest and make thegrains of the aluminum at the surface immobile. Immobilizing the grainscauses a reduction in movement across grain boundaries and a suppressionof grain growth. This in turn inhibits voids from forming and prohibitsvoids that do form from coalescing to form large-scale voids. The resultis an aluminum film with a reduced amount of voids and notches from theprior art and with a reduced coalescence of the voids that are present.

In a second embodiment, the passivation species is deposited with CVD asorganometallic chemical vapor deposition (MOCVD). The passivationspecies used in the second embodiment may comprise, for instance, Ti,TiN, TiC_(x),N_(y),TiC_(x),N_(y),O_(z), Cu, Sc, C, Si, Ge, and Sb. TheMOCVD deposited passivation species achieves surface passivation asdescribed above, and in addition, forms an alloy with the adjoiningaluminum films, thereby improving electromigration resistance.

A third embodiment involves multiple depositions of aluminum. Under thisembodiment, an aluminum film having a first thickness is deposited andthen passivated in the manner discussed above. An aluminum film having asecond thickness is deposited and is also passivated. Additionalaluminum films are then subsequently deposited, each having apredetermined thicknesses. A layer of passivation species is depositedafter each aluminum film deposition, and the process is repeated untilthe final desired thickness of a resultant composite aluminum film isobtained. Additionally, each subsequent deposition is preferablyconducted at a successively lower temperature, further helping tosuppress void formation and to reduce coalescence of voids intolarge-scale voids.

An aluminum film structure is produced by the method of the presentinvention, having a substrate assembly and a plurality of adjacentlayers situated upon the substrate assembly. Each adjacent layer has asubstantially uniform thickness and an upper surface The upper surfacehas a plurality of aluminum grains that are substantially immobile suchthat the grain boundary movement thereof is substantially arrested. Eachadjacent layer has substantially no voids larger than the thicknessthereof.

The aluminum film produced by the method of the present invention ishighly suitable for forming metal interconnect lines and for forminginter-level interconnect structures down to semiconductor devices on asubstrate assembly. The aluminum film has less voids and notches than ifthe passivation species were not used, and any voids and notches that doexist will not cross over the boundaries between the discrete layerslaid by the multiple deposition steps. This in turn limits the maximumdiameter of voids that can occur to the thickness of any of the discretelayers.

The above discussion illustrates several embodiments of a method forforming aluminum films. The method is suitable for use in formingaluminum interconnect lines, contacts, vias, and for other aluminum filmapplications. The method provides an interface with reduced occurrenceand size of voids and notches, reduced electromigration, and increasedconductivity. Faster device speeds and improved yield rates are also aresult of the inventive method.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesof the invention are obtained will be understood, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which is illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be considered tobe limiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a cross-sectional depiction of a contact opening filled by analuminum film on an in-process integrated circuit wafer, showing theoccurrence of voids within the aluminum film of the contact opening.

FIG. 2 is a cross-sectional depiction of the contact opening of FIG. 1after higher temperature processes have been conducted and showing thecoalescence of the voids into a larger void.

FIG. 3 is a cross-sectional depiction of a film of aluminum depositedunder a method of the prior art and showing voids at the interfacebetween aluminum and titanium nitride layers.

FIG. 4 is a cross-sectional depiction of an aluminum film depositedunder a basic embodiment of the method of the present invention.

FIG. 5 is a cross-sectional depiction of aluminum film deposited under asecond embodiment of the method of the present invention.

FIG. 6 is a cross-sectional depiction of a contact opening filled withan aluminum film in accordance with the second embodiment of the methodof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of the present invention will be described in severalembodiments illustrated in FIGS. 4 through 6 of the accompanyingdrawings. The first step of the present invention is illustrated in FIG.4 and comprises the deposition of an underlying layer 30 on a substrateassembly 10. Substrate assembly 10 typically comprises a substrate uponwhich may be formed one or more structural layers. In the depictedembodiment, substrate assembly 10 comprises a silicon substrate of anin-process integrated circuit wafer. Underlying layer 30 preferablycomprises titanium nitride, and in the depicted embodiment, is formedover a passivation layer 22 of BPSG or SiO₂.

When the aluminum film is being used to form a contact or via, as in theembodiment of FIG. 6, a contact opening 52 is first formed throughpassivation layer 22 down to an underlying active region, semiconductordevice, or other such surface which otherwise must be electricallyinterconnected with a surface layer. In such an application, underlyinglayer 30 is then deposited in contact opening 52 to serve as a diffusionbarrier.

Returning to the planar film embodiment of FIG. 4 for discussionpurposes, after underlying layer 30 is deposited, an aluminum film 32having an initial thickness of a fraction of the final desired thicknessof aluminum film 32 is deposited over underlying layer 30. Aluminum filmdeposition is preferably conducted with a cold wall CVD process such asthat described above, though it should be apparent to one skilled in theart that any suitable deposition process could be used. The thickness ofaluminum layer 32 is preferably about one third to one half of the finalthickness of the aluminum film which is desired to be deposited.

In a further step, the surface of aluminum film 32 is exposed to aspecies which achieves surface passivation. The passivation species ispreferably deposited as a passivation layer 36 which coats at least thetop surface of aluminum film 32. Passivation layer 36 can also beallowed to penetrate into aluminum film 32 to pacify portions ofaluminum film 32 beyond the top surface.

Examples of suitable passivation species comprise O₂, N₂, TiCl₄,tetradimethylaminotitanium (TDMAT) SiH₄, GeH₄, along with members of thehalogen and interhalogen families of gases. The passivation species isdeposited in such a manner as to at least partially coat the aluminumgrains. This will allow the passivation species to precipitate in thegrain boundaries, and thereby cause friction and the freezing in placeof the grains of aluminum film 32. Further grain growth duringsubsequent aluminum film deposition is arrested as a result. Arrestingthe movement of the grains of aluminum film 32 substantially reduces theformation of voids as well as the coalescence of voids into large-scalevoids.

An aluminum film 34 having a second thickness is then deposited overaluminum film 32. The second thickness of aluminum film 34, in the basicembodiment of the invention, makes up the balance of the final thicknessof the aluminum film which is desired to be deposited.

Alternative embodiments are also provided in the present invention. Inone alternative embodiment, the passivation species comprises a materialsuch as Ti, Cu, C, TiN, TiC_(x) N_(y), TiC_(x) N_(y) O_(z), Si, Ge, Sc,and Sb, etc. which combines and alloys with the aluminum to not onlypassivate the surface, but also to reduce electromigration. Thepassivation species in this embodiment is preferably deposited with anorganometallic chemical vapor deposition (MOCVD) process. Thus, underthis embodiment, underlying layer 30 and aluminum film 32 are depositedas discussed above in reference to the embodiment of FIG. 4, andpassivation layer 36, selected from the aforementioned alternate groupof materials, is deposited with MOCVD. As a consequence, when depositingsecond thickness of aluminum 34, the selected passivation species willalloy into second thickness of aluminum, and to a lesser extent, intoinitial aluminum film 32, thereby causing increased electromigrationresistance in both films.

In a further alternative embodiment of the method of the presentinvention, the deposition of aluminum is conducted in multiple steps.The initial thickness of aluminum is deposited at a higher temperature,and each deposition step is conducted at a successively lowertemperature. This causes a reduction in size of the grains of thealuminum film, thereby reducing voiding and film stress and preventinggrain growth in the portions of the aluminum film which have beendeposited during prior steps.

This embodiment is illustrated in FIG. 5. As seen in FIG. 5 therein, aninitial aluminum film 38 is deposited over underlying layer 30. Apassivation species in the form of passivation layer 40 is thendeposited over initial aluminum film 38 in order to suppress theformation and coalescence of voids. Subsequently thereafter, successivealuminum films represented in FIG. 5 as aluminum films 42, 44, and 50are deposited with intervening passivation layers 44 and 48. Thepassivation species used to form passivation layers 40, 44, and 48 canbe the same or may vary according to the inventive method.

In one example, given by way of illustration and not intended to berestrictive, initial aluminum film 38 is deposited with a thickness ofabout 1000 angstroms at a temperature of about 250° C. Successivealuminum films 42, 44, and 50 are deposited with a thickness of about200 angstroms at a temperature of about 150° C. Successive depositionsare conducted with intervening passivation layer deposition until theaccumulated thickness of aluminum reaches about 3000 angstroms.

FIG. 6 illustrates a further embodiment of the method of the presentinvention. In the embodiment of FIG. 6, an inter-level interconnectstructure in the form of a contact opening 52 is metallized with analuminum film formed under the method of the present invention. Contactopening 52 electrically interconnects an active device 10a, which in thedepicted embodiment comprises a junction of a MOS semiconductortransistor. Under this embodiment, contact opening 52 is metallized inmultiple steps in accordance with the discussion of FIG. 5, whereinaluminum films 38, 42, 46, and 50 are deposited with the successiveintervening deposition of surface passivation layers 40, 44, and 48 toresult in the inter-level interconnect structure of FIG. 6.

The size of voids that can occur in the resulting contact structure ofFIG. 6 are limited to the thicknesses of each of aluminum films 38, 42,46, and 50. That is, passivation layers 40, 44, and 48 will prohibitcoalescence of voids that intersect the boundaries between aluminumfilms 38, 42, 46, and 50. This in turn limits the maximum diameter ofthe voids that can occur in contact opening 52 to a diameter of the samedimension as the thickness of any one of aluminum films 38, 42, 46, and50. Such is also the case when planar films are formed as in thestructures of FIGS. 4 and 5. Thus, if each of aluminum films 38, 42, 46,and 50 are of uniform thickness and together fill contact opening 52,substantially no voids will result that have a diameter exceeding thediameter of contact opening 52 divided by the number of aluminum films,in this case, four.

The above discussion illustrates several embodiments of a method forforming an aluminum film. The aluminum film is suitable for forminginter-level interconnect structures such as aluminum interconnect lines,contacts, vias, as well as other aluminum film structures where a needexists for a reduced occurrence of voids and notches, reducedelectromigration, and increased conductivity. The inventive method hasbeen found to promote faster device speeds as well as improved yieldrates as compared to conventional aluminum film deposition.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrated andnot restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. An inter-level interconnect structure on a substrateassembly comprising:a substrate assembly; and a plurality of adjacentaluminum layers situated upon said substrate assembly each having asubstantially uniform thickness and an upper surface, each said uppersurface having a plurality of aluminum grains that are substantiallyimmobile such that substantially no grain boundary movement occurs,wherein each adjacent aluminum layer has a cylindrical cross-section andsaid plurality of adjacent aluminum layers situated upon said substrateassembly have concentric cylindrical cross-sections, said concentriccylindrical cross-sections forming an inter-level interconnect structureproviding electrical communication to a buried semiconductor device onthe substrate assembly.
 2. The inter-level interconnect structure asdefined in claim 1, wherein the plurality of adjacent aluminum layersare situated upon a refractory metal nitride layer that is in contactwith and situated upon said substrate assembly.
 3. The inter-levelinterconnect structure as defined in claim 1, wherein each adjacentaluminum layer has a substantially uniform thickness of less than onemicron.
 4. The inter-level interconnect structure as defined in claim 1,wherein each adjacent aluminum layer has therein substantially no voidslarger than about the thickness thereof.
 5. The inter-level interconnectstructure as defined in claim 1, wherein an interface is situatedbetween each adjacent aluminum layer of said plurality of adjacentlayers, and wherein substantially no voids are in contact with theinterface between each adjacent layer of said plurality of adjacentaluminum layers.
 6. An aluminum film structure on a substrate assemblycomprising:a substrate assembly; a refractory metal nitride layer uponsaid substrate assembly; a plurality of adjacent aluminum layerssituated upon said refractory metal nitride layer each having asubstantially uniform thickness of less than one micron and an uppersurface, said upper surface having a plurality of aluminum grains thatare substantially immobile such that substantially no grain boundarymovement occurs, and each having therein substantially no voids largerthan about one micron, wherein an interface is situated between eachadjacent aluminum layer of said plurality of adjacent layers, andwherein substantially no voids are in contact with the interface betweeneach adjacent layer of said plurality of adjacent aluminum layers. 7.The aluminum film structure as defined in claim 6, wherein each adjacentaluminum layer has a cylindrical cross-section and said plurality ofadjacent aluminum layers situated have concentric cylindricalcross-sections, said concentric cylindrical cross-sections forming aninter-level interconnect structure providing electrical communication toa buried semiconductor device on the substrate assembly.
 8. Aninter-level interconnect structure comprising:a substrate assembly; aburied semiconductor device on the substrate assembly; a plurality ofadjacent concentric cylinders providing electrical communication to theburied semiconductor device, each of the plurality of concentriccylinders having a submicron sized and substantially uniform radius, andwherein substantially no voids exist that are greater than about therespective diameter of any of the plurality of adjacent concentriccylinders, and wherein substantially no voids exist between adjacentconcentric cylinders that are greater thin about the respective diameterof any of the plurality of adjacent concentric cylinders.
 9. Theinter-level interconnect structure as recited in claim 8, wherein theplurality of adjacent concentric cylinders contain substantially novoids of a diameter greater than a size corresponding to the diameter ofthe plurality of adjacent concentric cylinders divided by the amount ofconcentric cylinders in the plurality of concentric cylinders.
 10. Theinter-level interconnect structure as defined in claim 1, wherein theburied semiconductor device on the substrate assembly is an active areawithin a silicon layer.
 11. The inter-level interconnect structure asdefined in claim 6, wherein the buried semiconductor device on thesubstrate assembly is an active area within a silicon layer.
 12. Theinter-level interconnect structure as defined in claim 8, wherein theburied semiconductor device on the substrate assembly is an active areawithin a silicon layer.
 13. An electrical connection structurecomprising:a substrate assembly; and a plurality of adjacent aluminumlayers each having the same thickness and having a cylindricalcross-section, wherein the cylindrical cross-section are concentric; aburied semiconductor device on the substrate assembly in electricalcommunication with said concentric cylindrical cross-sections.
 14. Theelectrical connection structure as defined in claim 13, wherein eachsaid aluminum layer has an upper surface having a plurality of aluminumgrains that are substantially immobile such that substantially no grainboundary movement occurs.
 15. The electrical connection structure asdefined in claim 13, wherein each adjacent aluminum layer has therein novoids larger than about the thickness thereof.
 16. The electricalconnection structure as defined in claim 13, wherein each adjacentaluminum layer has a uniform thickness of less than one micron.
 17. Analuminum film structure comprising:a substrate assembly; a refractorymetal nitride layer upon said substrate assembly; a plurality ofadjacent aluminum layers situated upon said refractory metal nitridelayer each having a substantially uniform thickness of less than onemicron, and each having therein substantially no voids larger than aboutone micron, wherein an interface is situated between each adjacentaluminum layer of said plurality of adjacent layers, and whereinsubstantially no voids are in contact with the interface between eachadjacent layer of said plurality of adjacent aluminum layers.
 18. Thealuminum film structure as defined in claim 17, wherein each adjacentaluminum layer has a cylindrical cross-section and said plurality ofadjacent aluminum layers situated have concentric cylindricalcross-sections.
 19. A contact structure comprising:a silicon layer; anactive area within the silicon layer; a plurality of adjacent concentricaluminum cylinders in electrical communication with the active area,each said cylinder having a submicron sized and uniform radius, andwherein the plurality of adjacent concentric aluminum cylinders havetherein a plurality of voids, wherein each said void is less that any arespective diameter of any of the cylinders.
 20. The contact structureas defined in claim 18, wherein any of said voids situated betweenadjacent concentric cylinders is not greater than the respectivediameter of any of the plurality of adjacent concentric cylinders. 21.The electrical connection structure as defined in claim 18, wherein eachadjacent aluminum cylinder has a uniform thickness of less than onemicron.